1. Field of the Invention
This invention is related to the field of integrated circuits and, more particularly, to leakage current detection in integrated circuits.
2. Description of the Related Art
Integrated circuits are manufactured using a semiconductor manufacturing process appropriate for the semiconductor substrate material being used and appropriate for the types of transistors being fabricated. For example, complementary metal-oxide-semiconductor (CMOS) transistors fabricated in silicon substrates are very popular in industry today. Other semiconductor substrates may include, e.g., galium arsenide (GaAs), silicon on insulator (SOI), etc. Other transistors may include bipolar junction transistors (BJTs), other types of field effect transistors (FETs), etc.
The semiconductor manufacturing process is complex, and subject to numerous adjustments that affect the performance of the transistors fabricated using the process. Processes are often described as running “hot” (transistors having more rapid switching qualities) or “cold” (transistors having less rapid switching qualities). A variety of specifications are agreed to between the foundry that implements the process and the designer that designs the integrated circuit.
One specification that has risen in importance as the feature sizes in processes have continued to shrink is the leakage current specification. Leakage current may generally refer to the current through a transistor when the transistor is logically inactive (or logically non-conductive) according to the control node voltage and/or control current of the transistor. For example, in CMOS transistors, the transistor is logically inactive/logically non-conductive if the gate voltage is lower than the source voltage (for N-type MOS (NMOS) transistors) or if the gate voltage is higher than the source voltage (for P-type MOS (PMOS) transistors). Leakage current may occur from a variety of sources. For example, in CMOS transistors, leakage current may occur through the oxide from the gate to source or drain, from the source to the drain (over a short channel), etc. There may be numerous sources of leakage current in a given transistor.
Leakage current is typically much smaller in magnitude than the magnitude of the active current that flows through a transistor when the transistor is logically active (or conductive), by several orders of magnitude. However, the leakage current contributes to the static power consumption of an integrated circuit and thus may contribute to high power consumption even though the magnitude of the leakage current is fairly small. Accordingly, a specification for low leakage current is desirable. Leakage current is affected by the various process parameters in the semiconductor manufacturing process, as well as by operating temperature during operation of the integrated circuit. Leakage current can vary across a semiconductor wafer as well.
Currently, the foundry supplies leakage current data for a given wafer to the designer. The leakage current is measured from small amounts of circuitry in the “scribe” area of the wafer (the area between chips on the wafer, where cuts are made during the dicing process to divide the wafer into individual chips). Unfortunately, the leakage current measurements are typically taken from relatively few sites on the wafer, and thus variations across the wafer may not be accurately captured. Additionally, there are physical differences between the scribe area and the chips themselves, which may impact the accuracy of the scribe leakage current measurements with respect to the chips. Still further, the scribe circuits are very small, and thus the magnitude of the leakage current being measured is also very small. The low magnitude increases the challenge of getting accurate leakage current measurements.